To an increasing extent, motor vehicles, such as motorized passenger vehicles, are equipped with safety systems such as front, side, knee and head airbags. In the event of a collision, the passengers are to be protected by these safety systems, and the risk of injury is to be reduced. Airbags must be unfolded and inflated within a very short length of time. For this purpose, propelling charges are used, which charges fill the airbag explosively and cause them to exit from the respective lining into the interior of the vehicle. The arrangement of the airbags and the selection of their sizes represent a compromise that is to be struck among the various sizes and various weights of the vehicle passengers. In the case of front airbags, there is also often provision to inflate the airbags at various levels depending on the seat positions of the vehicle passengers. Thus, in the case of a tall passenger, whose vehicle seat is arranged correspondingly farther away from the dashboard, a front airbag is to be inflated more fully than in the case of a shorter vehicle passenger, whose vehicle seat is shifted into a position that is closer to the dashboard. This is to prevent a vehicle passenger who is seated closer to the dashboard from being injured by the impact of an airbag that is inflated at full power. The inflation energy for the airbag is correspondingly controlled by using graduated amounts of propelling charges that are ignited. For the control of the inflation energy for the airbag, it is therefore important to know the approximate distance between the vehicle seat and the dashboard. In this case, it does not result in an exact measurement of distance; it is sufficient when, for example, two states of the vehicle seat, namely forward or rearward, can be detected.
In the past, therefore, different mechanical or electromechanical systems had been used to determine the position of the vehicle seat. Mechanical or electromechanical detector systems are, however, susceptible to wear and tear and can lead to unpleasant, undesirable noises when the vehicle seat is adjusted.
In the course of increasing automation, motor vehicles are being equipped more and more with electrical and electronic components that take over the function of earlier mechanical or electromechanical sensor systems. Thus, from the state of the art, non-contact sensor systems are known, with which the relative positions of two components that can move toward one another can be detected in order to generate a corresponding control signal therefrom. In the case of the vehicle seat, the components that can move relative to one another are, for example, a guide rail mounted on the vehicle bottom and a seat rail securely connected to the vehicle seat, which seat rail can be moved in a linear manner along the guide rail. In order to be able to determine the relative positions of the two rails, a magnetic strip can be applied to, for example, the guide rail, along which a Hall sensor that is connected to the seat rail can be moved. The magnetic strip can, as described in U.S. Pat. No. 4,909,560, change its polarity multiple times along its longitudinal extension. Upon relative movement along the magnetic strip, the output signal of the Hall sensor varies as a function of the immediately detected magnetic pole. This makes possible an incremental detection of the relative position of the vehicle seat.
A position sensor based on a Hall sensor, known from DE 101 36 820, allows the detection of two seat positions, forward and/or rearward, corresponding to a small or a large distance of the vehicle seat from the dashboard. In order to achieve the largest possible Hall sensor signal that can be analyzed, both publications propose keeping the distance between the magnetic poles and the surface of the Hall sensor as small as possible. In connection with the usual manufacturing and mounting tolerances, this can, however, cause the Hall sensor or its housing to grind against the guide rail when the seat rail is moved. Aside from the undesirable noise produced and the increased shifting resistance, this grinding contact can result in damage and in a failure of the sensor system.
A sensor arrangement that is mounted on the seat rail and that monitors a query plate mounted on the guide rail is known from JP 2003-227703. This sensor arrangement includes a Hall sensor, a preloaded magnet, and a flux guide plate, which are mounted inside a housing. For example, the housing has a U-shaped configuration with a take-up gap for the query plate that is to be monitored. The Hall sensor, the preloaded magnet, and the flux guide plate can be arranged on both sides of the take-up gap. An alternative variant embodiment provides that all components of the sensor arrangement are arranged on one side of the take-up gap. The flux guide plate serves to concentrate the magnetic flux to the Hall sensor and, moreover, is to shield against disruptive influences from external magnetic fields. When the vehicle seat is moved from a “rearward” position into a “forward” position, the query plate ends up in the take-up gap of the housing of the sensor arrangement. As a result, the magnetic flux through the Hall sensor is changed and generates a signal that can be assigned to a seat position. With this sensor arrangement the housing for the sensor arrangement is relatively large and has to be arranged very exactly in terms of the query plate. Also, the query plate has to be mounted separately on the guide rail, which increases the mounting costs.
The forward free end of the guide rail that points toward the footwell is, moreover, often provided with a cover, so that the danger of damage to the guide rail is eliminated. The cover can now cause the housing for the sensor arrangement to have to be mounted projecting from the seat rail relatively far to the side, so that it does not hinder the movement of the seat rail along the guide rail. The effect of this is also that the query plate that is mounted on the guide rail has to project relatively far to the side, so that it can be accommodated when it runs over the take-up gap of the housing of the sensor arrangement. In turn, however, the query plate that projects relatively far to the side can lead to impediments, for example when an object slides laterally under the vehicle seat. In this case, the danger exists that the query plate will become bent, which can impair the seat adjustment or can make correct detection of the seat position impossible, since, for example, the change in signal is no longer large enough.
With known sensor systems, in the event of an incorrect mounting, or in the event that the sensor system drops off the rail system, the sensor systems may not have any clearly defined state; in the emergency of a collision, this can lead to an additional risk for a passenger sitting in the vehicle seat if the airbag is inflated with an energy that is too low for the occupied seat position and for the height of the passenger.